1. Field
This relates to power sources, including for example constant current sources such as electronic ballasts that may be used with gas discharge lamps such as fluorescent lamps, and similar power sources, including for example ones in which power from the output is fed back to the input in order to achieve power factor correction of the input current.
2. Related Art
In electronic ballast-types of power sources, AC power is supplied to a load such as a fluorescent lamp in a constant-current mode to insure reliable and adequate production of light in the lamp. Incoming current is taken from a line supply, rectified and applied to an inverter to generate a high frequency alternating current signal. The high frequency alternating current signal is applied at an output to a transformer that induces a constant current signal in a lamp circuit for driving the lamp or lamps. If the AC current on the incoming line is not in phase with the voltage, then current is drawn from the utility lines at times when there is not proportionate voltage, typically resulting in wasted power. In such situations, the power factor of the circuit is less than optimum (1 or unity), for example significantly less than 95 percent. Additionally, total harmonic distortion (THD) is high which can result in overheating of power line neutral conductors in big buildings.
Some conventional electronic ballasts apply power factor correction to improve efficiency and reduce total harmonic distortion. Power factor correction can be accomplished using a boost circuit, though other circuits can also be used for power factor correction. With a boost circuit, however, expensive transistors, inductors and control chips are used, resulting in higher costs. In a charge pump boost circuit, power factor correction may be provided at a relatively low added cost, but the resulting circuit may be inefficient for example because of a large circulating current. Additionally, some ballasts incorporating charge pump boost circuits may be damaged if lamps are removed while the ballast is operating. Protection circuits added to the ballast circuit may make the ballast more complex or inefficient, which mitigates the cost advantages of the resulting product.